For short-range communication, near-field communication techniques are increasingly being used. In comparison with conventional (far-field) radio wireless transmission, in which the intensity of a transmitted electromagnetic signal falls with the square of the distance, in the near field the intensity of the transmitted signal falls with the cube of the distance. Moreover, the interference, attenuation and reflection properties of interposed materials and objects are very different for near-field transmission relative to far-field electromagnetic transmission. For operating frequencies in the megaHertz range, the near-field properties dominant over the range of decimeters (dm) to a few meters (m), so short-range wireless communications such as body area networks operating at these frequencies, are suitable as near-field communication systems. Near-field magnetic induction is known for use in hearing aids, and in particular for retransmission of an audio signal to monophonic or stereophonic hearing aids.
Where a network comprises two or more transmission sources which are in sufficiently close proximity that there can be interference between them, multiplexing methods are required to ensure adequate signal reception. One known such multiplexing method is time division multiplexing access (TDMA). According to the system, difference timeslots within a “frame” are allocated, or scheduled, to different transmitters in the network. Typically, the frame may comprise one or more random channel timeslots, together with two or more synchronous channel timeslots. The random channel timeslots are used for broadcasting control signals such as accommodation signals and notifications as to which transmitter may use individual synchronous channel timeslots. Since these control signals have to be disseminated to all the receivers, they are also termed beacon signals. Random channel slots are available to any transmitter channel, and thus there may be the potential for conflicting concurrent transmission from more than one transmitter. In contrast, during the synchronous channel timeslots, a data signal (such as, for instance, an audio stream) is transmitted only by the nominated transmitter, whilst the other transmitters remain silent so as not to produce interference. Such a time-multiplexed magnetic induction communication system is disclosed in U.S. Pat. No. 5,982,764.
The number of synchronous channel timeslots within any frame is typically fixed, and to ensure that a frame is not overly long, the number of synchronous channel timeslots is limited. It would be desirable if more timeslots could be made available, in order to reliably improve the band-width of the system.